The present invention relates to a technique for reducing a power loss and heat generation in a discharge lamp lighting circuit and for guaranteeing stable lighting control.
There has been known a lighting circuit for a discharge lamp (a metal halide lamp) comprising a DCxe2x80x94DC converting circuit, a DC-AC converting circuit and a starting circuit (a so-called starter circuit). Assuming that the number of discharge lamps is one, for example, there is a structure comprising a DCxe2x80x94DC converter, a half bridge type circuit (a circuit constituted to make a set of two semiconductor switching elements and to alternately carry out ON/OFF control) and a control circuit thereof. The output of the converter is controlled by the control of the switching element in the DCxe2x80x94DC converter and an AC output generated by the alternating operation of the switching element constituting the half bridge type circuit is supplied to the discharge lamp. The DCxe2x80x94DC converter includes a configuration in which a circuit portion corresponding to each polarity is provided (a voltage converting circuit corresponding to the polarity is completely isolated) in order to obtain output voltages having positive and negative polarities and a configuration in which outputs having both polarities can be obtained with one structure without isolating the circuit portion.
Moreover, in the case in which the number of the discharge lamps is two, there is a disadvantage in that the number of components and a cost cannot be reduced if respective lighting circuits corresponding to the discharge lamps are provided to light up the discharge lamp. Therefore, it is preferable that the lighting circuits for two discharge lamps are made common. For example, there is a structure comprising a DCxe2x80x94DC converter and a full bridge type circuit (a circuit constituted to have four semiconductor switching elements in pairs and to alternately carry out ON/OFF control for two pairs of elements) and a control circuit thereof. More specifically, the output of the converter is controlled by the control of the switching element in the DCxe2x80x94DC converter. In addition, when a rectangular wave-shaped output generated by the alternating operation of the switching element constituting the full bridge type circuit is to be supplied to the discharge lamp, an output having a positive polarity and an output having a negative polarity are alternately supplied to the discharge lamps (In other words, when the output having a positive polarity is applied to one of the discharge lamps, the output having a negative polarity is applied to the other discharge lamp). The DCxe2x80x94DC converter for obtaining output voltages having a positive polarity and a negative polarity includes the circuit structures having two configurations as described above.
In order to reliably light up the discharge lamp, it is preferable that a current auxiliary circuit including a capacitor should be provided in the subsequent stage of a DCxe2x80x94DC converter (for example, JP-A-9-223591). In the case in which the discharge lamp breaks down, the stored energy of the capacitor is supplied to the discharge lamp so that a transition to an arc discharge can be carried out smoothly.
FIG. 12 schematically shows a voltage (the output of the DCxe2x80x94DC converter, an upper part indicating a voltage having a positive polarity, a lower part indicating a voltage having a negative polarity and a current xe2x80x9cILxe2x80x9d in a middle stage indicating the current of the discharge lamp) which is supplied when one discharge lamp is lighted up on the assumption of a lighting circuit for the discharge lamp. The current auxiliary circuit is provided for the output having a positive polarity of the DCxe2x80x94DC converter, and a voltage output from the converter (a so-called open circuit voltage xe2x80x9cO.C.Vxe2x80x9d) is set to 350 V (volts), a limitation for the output having a negative polarity of the DCxe2x80x94DC converter is set to 150 V and a rated voltage of the discharge lamp is set to 85 V.
Assuming that the output having a positive polarity is applied to the discharge lamp in a certain timing, the output having a negative polarity has no path through which a current flows and is set in a non-load state. At this time, therefore, the voltage (magnitude) of the output having a negative polarity through the DCxe2x80x94DC converter is limited to 150 V.
When the polarity is inverted in a next timing and the output having a negative polarity is applied to the discharge lamp, the output having a positive polarity has no path through which a current flows and is set in a non-load state at this time. In this case, therefore, the voltage (magnitude) of the output having a positive polarity through the DCxe2x80x94DC converter is set to 350 V.
The operation described above is repeated according to the operating frequency of a bridge circuit (a frequency at which the switching operation of the switching element is alternated) so that an output having each polarity is supplied to the discharge lamp.
With the structure described above, however, the following drawbacks might be caused, for example.
(1) The influence of an AC component (based on a pulsating flow) with a low lighting frequency hinders constant power control;
(2) An increase in an invalid power component; and
(3) An increase in a loss in a current auxiliary circuit.
As shown in FIG. 12, the current xe2x80x9cILxe2x80x9d flowing to the discharge lamp takes a waveform having a pulsating flow.
The reason is that a smoothing circuit is provided in the output stage of the DCxe2x80x94DC converter and electric charges accumulated in a smoothing capacitor are mainly discharged in a non-load state. In other words, the energy of the smoothing capacitor which is stored to have a voltage of approximately 350 V or xe2x88x92150 V is discharged to the discharge lamp to have a voltage of 85 V or xe2x88x9285 V during the polarity inversion of the bridge type circuit. Therefore, a current is increased immediately after the polarity inversion and the pulsating flow causes a great fluctuation in an output voltage, resulting in bad effects (1) to (3).
First of all, referring to the (1), the current to flow to the discharge lamp is temporarily increased without any control instruction instantaneously when the polarity of the discharge lamp is switched. Therefore, the control circuit tries to detect the increase to lessen the current flowing to the discharge lamp. However, when the capacity of the smoothing capacitor is large and the current is great during polarity switching, a higher power than a necessary control power is consequently applied. In order to avoid the application, for example, there is proposed a method of providing a delay (a delay circuit) in a circuit for detecting a value of a current flowing to a discharge lamp or a circuit for constant power control, thereby giving a response slowly. Consequently, a cost is increased. By slowly giving a response, moreover, there is a possibility that a power input (voltage) might be suddenly changed or a follow-up property for a change in the condition of the discharge lamp might be deteriorated. Thus, a possibility that the discharge lamp might go out is increased, which is not preferable.
Referring to the (2), moreover, description will be given with a specific numeric value. For example, when a rated power is set to 35 W, an electrostatic capacity of the smoothing capacitor is set to 0.47 xcexcF and a lighting frequency of the bridge type circuit is set to 1 kHz, a DCxe2x80x94DC converter for an output having a positive polarity stores electric charges in the smoothing capacitor every 1 mS (millisecond) from 85 V (rated voltage) to 350 V (the O.C.V). An energy is xe2x80x9c0.47xc3x9710xe2x88x926xc3x97(3502xe2x88x92852)=0.054Jxe2x80x9d, which is a power per second of 0.054Jxc3x971000 (times)=54W. In order to set the power per second to a steady-state power of the discharge lamp or less, accordingly, it is preferable that the capacity of the smoothing capacitor or the lighting frequency should be regulated. As described above, a distorted AC component gives an invalid power component to generate a loss which is a burden to a heat radiation design.
Referring to the (3), the output voltage of the DCxe2x80x94DC converter is raised or dropped with the lighting frequency of the bridge type circuit. For example, therefore, in the case in which a resistance element is connected to the capacitor in the current auxiliary circuit, a Joule heat loss is generated by a current flowing to the resistance element.
In the above case, one discharge lamp is provided. Also in the case in which the number of the discharge lamps is two, the same bad effects are produced.
More specifically, in the case in which one of the discharge lamps is lighted up and the other discharge lamp is not lighted up, the output voltages of the DCxe2x80x94DC converter in the outputs having positive and negative polarities act in the same manner as in the case in which one discharge lamp is provided. When the output having a positive polarity (or a negative polarity) is applied to the discharge lamp which is lighted up, the negative electrode side (or the positive electrode side) is brought into a non-load state. Accordingly, in the case in which two discharge lamps are provided and one of the discharge lamps is not lighted up, the same problems as described above might arise.
The invention has an object to reduce a possibility of element failures caused by a power loss and heat generation and to implement stable power control in a discharge lamp lighting circuit.
In order to solve the problems described above, in the invention, when alternately applying an output voltage of a DCxe2x80x94DC converting circuit to one discharge lamp, an output voltage on a non-load side in outputs having positive and negative polarities which is not applied to the discharge lamp during light-up of the discharge lamp is restricted to be equal to an output voltage on a side which is applied to the discharge lamp. Moreover, when supplying a power to two discharge lamps while alternately switching a polarity of an output voltage through the DCxe2x80x94DC converting circuit, an output voltage on a non-load side in the output shaving positive and negative polarities which is not applied to the discharge lamp is restricted to be equal to the output voltage on the side which is applied to the discharge lamp if one of the discharge lamps is lighted up or an instruction for light-up is given to the discharge lamp and the other discharge lamp is not lighted up or the instruction for light-up is not given to the discharge lamp.
According to the invention, therefore, the output voltage on the non-load side is limited to the output voltage having a reverse polarity thereto. Consequently, it is possible to suppress an AC component through a pulsating flow related to a current flowing to the discharge lamp.